Experimental and theoretical characterization of ordered MAX phases Mo2TiAlC2 and Mo2Ti2AlC3

Abstract

Herein, we report on the phase stabilities and crystal structures of two newly discovered ordered, quaternary MAX phases-Mo 2 TiAlC 2 and Mo 2 Ti 2 AlC 3 -synthesized by mixing and heating different elemental powder mixtures of mMo:(3-m)Ti:1.1Al:2C with 1.5 m 2.2 and 2Mo: 2Ti:1.1Al:2.7C to 1600 C for 4 h under Ar flow. In general, for m ! 2 an ordered 312 phase, (Mo 2 Ti)AlC 2 , was the majority phase; for m < 2, an ordered 413 phase (Mo 2 Ti 2 )AlC 3 , was the major product. The actual chemistries determined from X-ray photoelectron spectroscopy (XPS) are Mo 2 TiAlC 1.7 and Mo 2 Ti 1.9 Al 0.9 C 2.5 , respectively. High resolution scanning transmission microscopy, XPS and Rietveld analysis of powder X-ray diffraction confirmed the general ordered stacking sequence to be Mo-Ti-Mo-Al-Mo-Ti-Mo for Mo 2 TiAlC 2 and Mo-Ti-Ti-Mo-Al-Mo-Ti-TiMo for Mo 2 Ti 2 AlC 3 , with the carbon atoms occupying the octahedral sites between the transition metal layers. Consistent with the experimental results, the theoretical calculations clearly show that M layer ordering is mostly driven by the high penalty paid in energy by having the Mo atoms surrounded by C in a face-centered configuration, i.e., in the center of the M nþ1 X n blocks. At 331 GPa and 367 GPa, respectively, the Young's moduli of the ordered Mo 2 TiAlC 2 and Mo 2 Ti 2 AlC 3 are predicted to be higher than those calculated for their ternary end members. Like most other MAX phases, because of the high density of states at the Fermi level, the resistivity measurement over 300 to 10 K for both phases showed metallic behavior. V C 2015 AIP Publishing LLC.

Order and disorder in quaternary atomic laminates from first-principles calculations

“…Hence, for at least V and Ta, the configurational entropy may decrease the free energy for the solid solutions, and make the disordered Ti2M2AlC3 energetically favourable and likely to be realized during synthesis. Bulk synthesis of Ti2Mo2AlC3 has shown structures where the Wyckoff 4f-site is mainly occupied by Ti and Wyckoff 4e-site mainly by Mo, 24 which is a configuration closely related to the fully ordered type A structure considered in this work. The predicted stable ordered Ti2W2AlC2 and close to stable ordered Ti2Cr2AlC3 of type A have not yet been identified experimentally.…”

“…In particular, solid solutions on the M-site include, e.g., (Ti,M)2AlC where M = V, Nb, Ta, Cr, [8][9][10][11] (V,M)2AlC where M = Nb, Ta, Cr, 9,12 (Cr,Mn)2AC where A = Al, Ga, Ge, 3,[13][14][15][16][17][18][19] (Ti,V)3AC2 where A = Al, Ge, 20,21 and (V,M)4AlC3 where M = Ti, Nb. 20,21 Adding a fourth element has also been demonstrated by realization of TiCr2AlC2, 22,23 V1.5Cr1.5AlC2, 12 TiMo2AlC2, 24,25 and Ti2Mo2AlC3, 24 which all are recent discoveries of chemically ordered quaternary MAX phases, with atomic layers composed of a single element only. This raises the question why certain combinations of M elements form layered chemically ordered MAX phases, while other combinations result in a solid solution.…”

“…A is kept equal to Al, since i) all chemically ordered MAX phase alloys reported to date include A = Al, 12,[22][23][24][25] i.e. this choice allows theoretical validation of previous experimental results as well as prediction of new alloys, and ii) to date, all known MXenes originate from chemical etching of Al from a MAX phase, 26 with the only exception of Mo2C, 27 i.e.…”

Magnetic MAX phases from theory and experiments; a review

“…Phase stability calculations at 0 K have been proven successful when it comes to verify existing [12,14,22,26,81,[84][85][86] and, more importantly, predicting new MAX phases and related materials [3,7,25,87]. When it comes to magnetic MAX phases, almost all calculations to date neglect effects from temperature.…”

Structural and optical properties of the recently synthesized (Zr3−x Ti x )AlC2 MAX phases

“…MAX phases have good metallic properties high thermal shock resistance, electrical and thermal conductivity, damage tolerance, plastical deformation, whereas they are relatively soft and machinable [2][3][4][5][6]. Alike ceramics, they exhibit high melting temperature, high elastic rigidity, low density, creep and wear resistance, high elastic stiffness, high oxidation and corrosion resistance and can be used to form 2D-materials (MXenes) [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].…”